Coatings, such as for example, coatings used in paper and board manufacturing, are known to be mainly manufactured in batches in mixing containers. In addition, continuously operated manufacturing processes are known. In both principles, the operation principle is that coating components i.e. raw materials, are dosed to the mixing container, in which container they are mixed to form a prepared coating.
The problem with known and typical batch operated manufacturing processes is, that to change the type of coating requires several containers and hours of preparation. Thus the space requirement of the batch operated manufacturing processes is large. In addition, when washing a batch operated process, a lot of water that contains coating will be produced, and its further treatment can in some circumstances be cumbersome.
The problem with known typical continuous processes is their limited scope in handling recipes compared to batch processes. In addition, the requirements for reproducibility and accuracy of component dosing increase the costs of a continuous arrangement.
In addition, the problem with known coating manufacturing processes is their tendency, to some extent, to mix air into the coating. For example, in curtain coating, the air content of the coating can be at the highest 0-0.25 percent by volume. Otherwise the air bound to the coating may cause uncoated patches in the material to be coated, for example, paper or board. In a multi layer curtain coating the significance of deaeration is still increased. Then, if there are for example three or four layers of coating, the coating used when forming each layer must be deaerated, at least to remove free air bubbles, before coating.
In known arrangements, several methods have been tried to remove the air from the coating. Deaeration of the prepared coating is mainly based on the use of centrifugal force i.e. in practice the use of hydrocyclones. The coating is fed to the hydrocyclones under a relatively high pressure, the pressure being typically 1-4 bar. In the cyclones described, the area of pressure difference is approximately 1-2 bar in practice. When the coating is under such pressure, air is dissolved into the coating, more specifically into the liquid used in the coating manufacture, such as water. Therefore, with a centrifugal air-separator, only free air in the coating can be removed, which in addition, diminishes when the pressure rises. When the pressure affecting the coating is again reduced, for example, at the coating station, the air dissolved in the coating is released and expands due to the change in pressure, thus causing problems in the operation of the coating station.
In addition, in Patent Application Publication WO 02/066739 a paste manufacturing method is disclosed, where the paste is manufactured by mixing the pigments and binder (latex) together in an open mixer, from which the mixture produced is led to the deaeration. Because in the solution disclosed in the cited publication the deaeration is done by spraying the pigment binder mixture in one or two stages, then according to the publication, the viscosity of the mixture must be under 500 mPas (Brookfield 100 RPM 20° C.), and preferably under 200 mPas. After the deaeration, a surface active agent and thickener is added in a closed space to the pigment and binder mixture. The publication discloses that the pressure in the deaeration chamber is approximately 0.05 bar. The problem with the deaeration method disclosed in the publication is that it is not suitable for pastes and mixtures having a viscosity too high for deaeration by spraying. Therefore, the method disclosed in the publication requires a separate storage container, into which the prepared paste can be fed when it cannot, for example, during a malfunction, be fed directly to the coating station because due to the high viscosity of the paste, it probably cannot be led back to the open mixer.